The principles of Doppler Laser vibrometry are well known to the person skilled in the art, the general description can be found for example at the web site of Polytec company, CA. Vibrometers based on this principle can remotely measure surface velocities, or vibrations, with high spatial resolution and over a broad frequency and amplitude range.
A coherent laser beam is projected on to the surface under investigation. Light scattered back from the surface is shifted in frequency by an amount proportional to the velocity of the surface (the Doppler effect). The instrument measures this frequency shift to produce an instantaneous velocity signal, which can subsequently be analyzed.
By adding coordinate control using scanning mirrors, a single point vibrometer sensor can be used to scan across a surface, gathering multi-point data from vibrating objects. See for example, vibrometers produced by company Metrolaser, CA, Laser Doppler vibrometers at: http://www.metrolaserinc.com/vibrometer.htm [2].
The most basic laser Doppler vibrometer is the one with a reference beam, see for example models LV-1100 and LV-1300 produced by Ono Sokki, Japan. Laser light released from the light source is split into two beams, one of which serves as an incident beam directed at the object under measurement, while the other serves as a reference beam fed back within the vibrometer. The beam reflected from the object experiences Doppler shift proportion to the vibrators velocity of the object. This beam is then made to interfere with the reference beam, which is given a frequency shift beforehand by the acouslooptic modulator, so that a beat frequency can be obtained. From this beat frequency signal, only the Doppler-shift component is singled out at the detector circuit and sent to the FM demodulator to be output as a voltage signal that is proportional to the vibratory velocity.
The disadvantage of the described above system is in rather low sensitivity of the signal detection, and further improvement of the detection scheme is required to provide the measurement on longer distances and to improve the reliability of the system.
An object does not always vibrate in one direction only. It max in fact vibrate in a complex manner in three-dimensional directions. The LV-3300 is a vibrometric system comprising three reference-beam laser Doppler vibrometer units. The system performs vector calculations is when signals are received from these three vibrometers to simultaneously measure the X-, Y-, Z-axis vibratory velocity and the direction in which the object under measurment vibrates. One of the three optical heads is arranged so that the direction of its incident light coincides with the direction (Z-axis) in which the object moves, while the other two are arranged so that the directions of their incident light (ZX and ZY) are specific angles away from the Z-axis. Signals from these angled optical heads represent the vibratory velocities and vibrations in the ZX and ZY directions. Consequently, beams reflected from all three optical heads contain signal components for the Z, ZX and ZY directions in which the object vibrates. Thus, we can measure the vibratory characteristics of the object for the X-, Y-, and Z-axis directions simultaneously, by inputting these three signals into a vector calculator.
The sensitivity of described above system is not very high, that's why the measuring is carried out on short distances, usually less than one meter to avoid signal fading due to air turbulence effects as well as random vibrations of various elements in the system. In multiple applications a remote vibration measurement is required. One of the examples is a target recognition based on measured vibration frequencies. A target can be hundreds or thousands of meters away from the detector system, and its vibration frequencies must be measured in order to distinguish friend or foe.